Carbon monoxide detector

ABSTRACT

A carbon monoxide (CO) detection unit incorporates a sensor which is a film or layer of Ni x Co 1-x O y  where x is from 0.1 to 0.9, e.g., spinel. The CO is detected by measuring the change in the electrical properties of the sensor. The detector can measure CO concentrations below 100 ppm and is capable of operating at room temperature and can be applied to domestic, industrial, medical and vehicular use.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/208,601 filed 29 Jul. 2002, which is a continuation under 35 U.S.C.111(a) of International Application No.: PCT/GB01/00334 filed 29 Jan.2001 and published in English under Publication No. WO 01/55710 A1 on 2Aug. 2001, which claims priority from Great Britain Application No.0002027.1 filed 28 Jan. 2000 and Great Britain Application No. 0002808.4filed 9 Feb. 2000, which applications and publications are incorporatedherein by reference.

The present invention relates to a method and an instrument fordetecting carbon monoxide.

Carbon monoxide is an odourless, colourless, tasteless poisonous gaswhich can be produced by incomplete combustion of hydrocarbons. Gasfired domestic equipment such as water heaters, gas fires etc. can giveoff carbon monoxide when they are not functioning properly or when theyare not properly maintained and checked. The emission of carbon monoxidecan be fatal and there have been people killed by carbon monoxidepoisoning in these circumstances, even levels of carbon monoxide belowthe lethal limit can cause headaches, nausea and illness due to carbonmonoxide poisoning.

Apart from domestic situations, carbon monoxide can be emitted inindustrial operations and is a component of the exhaust of internalcombustion engines. The levels of carbon monoxide is a measure of airpollution and legal controls are imposed on carbon monoxide levels inair.

For these reasons accurate reliable carbon monoxide monitors are neededwhich can detect and monitor accurately low levels of carbon monoxide.

Existing types of carbon monoxide detectors can be broadly classifiedinto one of four types according to the gas sensitive element employed:chemical, electrochemical, semiconducting or spectroscopic (infra-red).The electrochemical and spectroscopic devices, whilst offering rapidresponse times, high resolution and high accuracy, are expensive and notsuitable for domestic use. Chemical sensors are inexpensive devices thatare usually based on palladium or iodine salts which exhibit a colourchange upon exposure to CO. They are of two classes; tapes forcontinuous monitoring which can provide very fast and sensitive response(typically sub ppm concentrations are sensed) but these require verycareful control over moisture content and tubes which are used for spotchecks and are of generally lower sensitivity than tapes although theydo not require such careful control of moisture. Both types rely on acolour change and could not be made “automatic” by the application of anelectronic device the degree of colour change. These devices are notreusable. However, their response to low CO concentrations tends to bepoor and therefore constant monitoring is required and can only be usedonce and fail to provide audible warning signals.

The most popular carbon monoxide detectors for domestic use utilise agas sensitive semiconductor, the resistance of which changes uponexposure to a reducing gas. Of these the most popular material is SnO₂and platinum doped SnO₂ other binary oxides include ZnO, TiO₂ and acombination of CuO and ZnO to form a heterocontact More recently the useof mixed metal oxide semiconductors for CO detection has been reported.These materials include the niobates CrNbO₄, FeNbO₄ andBa₆Fe_(1.5)Nb_(8.5)O₃₀ and the perovskite Lao_(0.5)Sr_(0.5)CO₃.

However, three problems exist with the use of semiconducting metaloxides for CO detection these are:

-   -   1. Low sensitivity to concentrations below approximately 100        ppm, this being equally as applicable to high operating        temperatures as it is to operation at room temperature.    -   2. Poor selectivity towards the gas desired for detection with        respect to cross sensitivity with co-existing gases.    -   3. Poor reproducibility of sensor characteristics, in particular        due to the fabrication processes employed. The use of ESD, and        related techniques, provides for greater control over        fabrication and therefore sensor characteristics.

SnO₂ provides resistance changes upon exposure to a wide range of gasesincluding, CO, CH₄ and H₂S. The problems of sensitivity and selectivityare commonly overcome by heating the semiconductor to approximately 200°C. for response to CO. However, such heating is considered undesirabledue to the power drain being too great for remote operation frombatteries and a mains power supply is necessary and sensitivity canremain poor with respect to sub 100 ppm levels even with heating togreater than 200° C.

We have now discovered a semiconducting oxide which exhibits appreciableresistance changes to CO concentrations below 100 ppm and is capable ofoperation at room temperature and which can be incorporated into COdetection units for domestic, industrial, medical and vehicular use.

According to the invention there is provided a method for the detectionof carbon monoxide in air which comprises contacting the air with asensor which incorporates an element formed from Ni_(x)Co_(1-x)O_(y) (I)where x is from 0.1 to 0.9, and y is 4x, and measuring the change in theelectrical properties of the element. Preferably, x is from 0.2 to 0.5and y is from 0.8 to 2.0. When x is 0.33 and y is 4x a spinel structureis formed. When x is not 0.33 some spinel will be present which will beactive but other oxides such as NiO and Co₃O₄ will also be present.

The invention also provides a device for detecting carbon monoxidelevels in air, which device comprises a sensor element comprising a filmor layer of (I) and a means able to measure a change in the electricalproperties of the film or layer on exposure to carbon monoxide.

Preferably (I) has a spinel structure and the stoichiometric formula of(I) is NiCo₂O₄, i.e., x is 0.33 and y is 4x, but in practice, (I) willhave a formula which can be expressed as Ni_(x)Co_(1-x)O_(y) withdeviation from the exact formula giving a sensor which is less effectivebut which could be useful. This is indicated by the change in resistancein air of the compound; the greater the resistance the greater thechange in resistance being needed for a reliable detection signal andthis is illustrated in FIG. 1 in which it can be seen that at X=0.33there is the lowest resistance.

Depending on the method of formation, a film or layer of (I) willcontain NiCD₂O₄ mixed with cobalt and nickel oxides to give acomposition in which the ratio of the components deviate from the strictstoichiometric ratios.

Any electrical property which changes on contact of the film with carbonmonoxide can be monitored e.g. the resistance, capacitance etc. Apreferred property is the resistance and the resistance of (I) increaseson exposure to carbon monoxide.

A device according to the invention which utilises the change inresistance of (I) comprises a substrate on which there is an elementwhich comprises a film or layer of (I), attached to the element areelectrodes and there is a means to measure the electrical resistance ofthe element. The means to measure the resistance can be any conventionalmeans, for example a small voltage is applied between the electrodese.g. from a battery and the current flowing monitored.

The film or layer of (I) can optionally include graphite powder,preferably in an amount of 5 to 20% of the weight of the film or layer.The graphite powder should be uniformly dispersed in the film or layerand preferably has an average particle size below one micron.

When the element is exposed to carbon monoxide the resistance of theelement increases and the current decreases, when the current decreasesbelow a pre-set level, which indicates a level of carbon monoxide abovea safe level, an alarm or warning can automatically be triggered.Alternatively the change in voltage at constant current can be used toevaluate the resistance change. In addition the device can be used toprovide for the continuous detection of carbon monoxide. The device canbe adapted to provide for the continuous monitoring of carbon monoxidelevels by means of circuitry which correlates the change in theresistance of the element with change in concentrations of carbonmonoxide.

If the power supply should fail, a warning or alarm is automaticallytriggered as the current drops and this a fail-safe feature againstinoperation due to power failure.

In order to measure the resistance there are electrodes attached to thefilm or layer of (I) and it is important that there is a good electricalcontact between the film and the electrodes so there is minimalresistance caused by the attachment of the electrodes to the element.

Preferred electrodes can be formed of an inert metal such as gold, whichhas no electrical junction effects with the sensor element. The contactmaterial itself should have no reaction with carbon monoxide.

In order to compensate for the effect on resistance of temperaturefluctuations a reference sensor can be used which is the same as thedetecting sensor but which is hermetically sealed from the atmosphereand so sealed from the sensing environment. Both the sensors would besubject to the same temperature fluctuations and so any difference intheir resistances would be due solely to the presence of a reacting gas.

As changes in the electrical resistance of the element are measured, alow base resistance of the element is preferred so the change inresistance of the element compared with the base resistance of theelement is greater. This can be accomplished by having an element with alow specific resistivity and there being a short path length through theelement between the electrodes. The resistance change required to give areliable signal will depend on the particular circumstances and theelectrical circuits used, but normally a resistance change of 25% issuitable although lower resistance changes can be used, particularlywhen a reference sensor is used to compensate for temperature variation.

The film or layer of (I) can be prepared by known methods for example bythermal decomposition of solutions of the metal nitrates or hydroxides,spray pyrolysis, cryochemical deposition, co-precipitation of the metaloxides and electrostatic spray deposition.

The preferred methods used are thermal decomposition and electrostaticspray deposition. However a printing technique, such as silk screenprinting may also be applied using NiCO₂O₄ powder and a solution ofmetal salts followed by thermal decomposition.

Electrostatic spray deposition is described in “A Review of LiquidAtomisation by Electrical Means” by J. M. Grace and J. C. M.Marijinissen, ESF Workshop on Electrospraying. Sevilla 1997 and“Electrostatic Spray Deposition of Doped YSZ Electrode Materials for aMonolithic Solid Oxide Fuel Cell Design” 10th IEA SOFC Workshop.Diablerets. Switzerland. 2 (1997) 236-247.

A preferred structure of the sensor of the present invention has a filmof (I) deposited or formed on to a substrate such as foil or ceramicsubstrate. Preferably a film of (I) is deposited on the substrate as itis formed e.g. by thermal decomposition of solutions of precursor metalsalts such as metal nitrates or by electrostatic spray deposition e.g.of precursor metal salts such as metal nitrates.

The nature of the substrate is not critical and any suitable substratecan be used, for example substrates which have physical propertiessimilar to those of the deposited film.

When thermal decomposition is used to form the film or layer, mixedcobalt and nickel nitrates are preferably thermally decomposed directlyonto a substrate such as a nickel foil. Nickel has a coefficient ofexpansion similar to (I) so the risk of cracking etc. due to changes intemperature is reduced. The film or layer can be made by forming a gelof cobalt nitrate and nickel nitrate in stoichiometric ratio byevaporation of a solution of the mixed nitrates on the substrate andthen drying and heating the gel at elevated temperature for example from250° C. to 650° C. e.g. 350° C. to form the film or layer of compound(I) on the substrate.

The technique of Electrostatic Spray Deposition (ESD), developed at theUniversity of Delft, provides an inexpensive and facile means ofpreparing thin films of inorganic substances. ESD has been successfullyapplied to production of a wide variety of thin films for solid oxidefuel cells, including those having a spinel structure.

ESD involves the production of an aerosol from a precursor solutionthrough the application of a high positive potential to a metalcapillary (nozzle), this being directed towards a heated, electricallygrounded substrate. As the precursor solution is pumped through thecapillary, droplets will grow at the tip of the nozzle.

The positive potential causes positive ions in the solution to migrateto the surface the droplet, resulting in a surface charge which causesan electrostatic pressure to oppose the surface tension of the liquid.Through the variation of electrode configuration, nozzle design andliquid properties (viscosity and conductivity) many spray modes, havingwidely different geometries, may be achieved.

A process of decomposition then occurs as charged droplets aresubsequently attracted by coulombic forces to the grounded substrate. Itis suggested that such decomposition occurs, together with solid statereaction of precursor cations, within the spray cone caused by the highpotential being applied

Conventional ESD can be used in the present invention.

Other techniques which can be used include flame assisted vapourdeposition and electrostatically assisted vapour deposition.

Another method of preparing NiCo₂O₄ films onto metal substrates is byco-spraying NiCo₂O₄ films powder and solution together via electrostaticspraying or simply depositing the mixture and solution onto thesubstrate followed by thermal decomposition e.g. at 400° C. or by use ofprinting techniques.

It is a feature of the sensors of the present invention that they areresistant to interference by other pollutant gases which are commonlypresent in the atmosphere such as oxides of nitrogen, referred to asNOx, and the sensors of the present invention work in the presence ofsuch gases.

In general the sensors work better at higher temperatures, but unlikeexisting sensors the sensors of the present invention detect low levelsof carbon monoxide at ambient temperatures. The sensors of the presentinvention are also relatively unaffected by changes in humidity.

The performance of the sensors of the present invention can be affectedby the addition of other metals as surface and bulk additives to thecompound (I). It was found that the addition platinum group metals e.g.palladium could improve the sensitivity, the levels of palladium usedpreferably range from 1 to 5% preferably 4 to 5%.

The invention is illustrated in the following Examples.

EXAMPLE 1

Nickel foil (99.9% Aldrich) had a film of (I) formed on it by a processin which the foil substrate was coated three times with mixed cobalt andnickel nitrates from an alcoholic solution, pressed under a 1 kg weightto equalise particle separation and obtain uniformity of coverage andthen fired at 400° C. for 2 hours between each successive coating. Theloading thereby obtained was 3.2 g/cm². The resistance was then measuredalong a linear profile on each surface via a two probe method using adigital multimeter (Hewlett Packard 3478A) and the contact spacingmeasured to two decimal places with a digital micrometer (Mitayoka). Therelationship between resistance and contact spacing was observed to belinear.

The sensors were evaluated for their response to carbon monoxide.Sensitivity is defined as (R_(g)-Ro)/Ro where R_(g) is the resistance inthe contaminant gas and Ro is the resistance in clean air.

A change in resistance of 25% was arbitrarily taken as a level whichwould be used to trigger an alarm. The sensor was exposed to aircontaining various levels of carbon monoxide and the time taken toregister a 25% change in resistance measured.

In exposure to air free of carbon monoxide the variation in resistanceover a long period of time (72 hours) was found to vary between 0.25 and0.6% of the initial value and in exposure to air with background levelsof carbon monoxide (1 ppm) the resistance was found to vary by −1.4 to+3.5% of the initial value thus indicating that there was little risk offalse positives being found.

The sensor was exposed to a carbon monoxide concentration of 10 ppm andthe resistance measured against time, a change in sensitivity of 3indicates a 25% change in resistance. The results and the result forexposure to background level of 1 ppm carbon monoxide is shown in FIG.2, as can be seen the warning level was attained after 75 mins. Existingsemiconductor sensors cannot detect carbon monoxide levels of this orderat ambient temperatures.

This was repeated for carbon monoxide concentrations of 35 ppm to 400ppm, 500 to 1000 ppm and for 5000 ppm to 2% and the results shown inFIGS. 3, 4 and 5.

The recovery of the sensor when the levels of carbon monoxide werereduced to background levels was also measured and the results shown inFIGS. 6, 7 and 8. As can be seen the resistance was quickly reduced toits base level.

The sensors were evaluated for sensitivity to NOx as a pollutant gaswhich is found some exhaust gases which contain carbon monoxide and itwas found that there was no decrease to the selectivity to carbonmonoxide on exposure to NOx in the concentrations likely to be found.

EXAMPLE 2 Electrostatic Spray Deposition

A ceramic substrate of 96% alumina having screen printed gold electrodes(Du Pont plc) was cut to the dimensions shown in FIG. 9 whichillustrates an example of a sensor substrate coated with NiCo₂O₄ usingelectrostatic spray deposition under a range conditions to prepare anumber of samples and the conditions used are shown in Table 1.

TABLE 1 Substrate Depo- Precursor Precursor Tem- sition Applied Sampleflow rate Molarity perature Time Potential Load- mg/cm² ml/hr. M ° C.hours kV ing 1 0.55 0.05 375 4 17.77 0.5 2 0.57 0.05 493 2 10.47 0.27 50.55 0.05 445 2 15.75 0.29 6 0.57 0.05 402 2 12.20 0.3 7 0.55 0.05 445 115.57 0.11 8 0.57 0.05 448 4 11.87 0.54 9 0.55 0.05 303 4 17.31 0.48 100.57 0.025 400 2 10.39 0.13 14 0.55 0.025 349 2 17.6 0.17 15 0.57 0.025373 2 9.82 0.14 16 0.55 0.05 393 2 17.19 0.29 17 0.57 0.025 372 2 10.230.14

The Solvent was 20 vol % absolute alcohol and 80 vol % butyl carbinol(Di(ethylene glycol)butyl ether, (99% Aldrich).

The resistance of the samples and the sensitivity to carbon monoxidelevels of 100 ppm for 1 hour as shown by the response time to achieve a25% increase in resistance were measured as described in Example 1 andthe results shown in Table 2

TABLE 2 Response Time Base Resistance Sample Mins. k ohms 1 60 1.52 2 454.24 5 20 4.20 6 50 2.88 7 20 4.36 8 30 2.22 9 20 1.40 10 40 2.52 14 304.60 15 30 2.44 16 30 1.12 17 20 0.28

As can b seen the sensors of the present invention can detect carbonmonoxide from low levels at ambient temperatures.

EXAMPLE 3

Sensor samples prepared using spray pyrolysis with and without theaddition of 5% palladium and the percentage increase in resistance after30 mins exposure time to 100 ppm of carbon monoxide was measured attemperatures from 50° C. to 200° C. and the results shown in Table 3which show sensitivity values.

TABLE 3 Sensor 50° C. 100° C. 150° C. 200° C. NiCo₂O₄ 9 7 10 13NiCo₂O₄ + Pd 8 18 26 36

As can be seen the addition of the palladium increases the sensitivity.

1. A device for monitoring carbon monoxide levels in air, wherein thedevice includes a sensor element having a film or layer comprisingNi_(x)Co_(1-x)Q_(y), where x is from 0.1 to 0.9 and y is 4x; whereinwhen carbon monoxide is detected, the resistance of the sensor elementincreases and the current through the sensor element decreases; and thedevice triggers an alarm or warning when the current decreases below apredetermined level.
 2. The device of claim 1 wherein x is from 0.2 to0.5 and y is from 0.8 to 2.0.
 3. The device of claim 2 wherein thesensor element comprises NiCo₂O₄.
 4. The device of claim 2 wherein thesensor element consists essentially of NiCo₂O₄.
 5. The device of claim 1wherein the sensor element includes a substrate comprising a film orlayer of Ni_(x)Co_(1-x)O_(y) and electrodes attached to the film orlayer.
 6. The device of claim 5 wherein the film or layer consistsessentially of NiCo₂O₄.
 7. The device of claim 6 wherein the electrodesare gold.
 8. The device of claim 7 wherein a voltage is applied to thesensor element and the current flow is monitored.
 9. The device of claim5 wherein the film or layer is formed by thermal decomposition ofsolutions of the metal nitrates onto a substrate.
 10. The device ofclaim 9 wherein the metal nitrates comprise a mixture of cobalt andnickel nitrates.
 11. The device of claim 10 wherein the film or layer ismade by forming a gel of cobalt nitrate and nickel nitrate in astoichiometric ratio by evaporation of a solution of the mixed nitrateson the substrate and drying and heating the gel at from 250° C. to 650°C. to form a film or layer having the formula Ni_(x)Co_(1-x)O_(y) on thesubstrate.
 12. The device of claim 11 wherein the substrate is nickelfoil.
 13. The device of claim 11 wherein the film or layer is formed byelectrostatic spray deposition.
 14. The device of claim 1 wherein whenthe level of carbon monoxide exceeds a predetermined level, the deviceemits an alarm or warning.
 15. The device of claim 11 wherein the filmor layer further comprises palladium as a surface or bulk additive. 16.The device of claim 15 wherein the sensor element comprises 1 to 5%palladium by weight.
 17. The device of claim 11 wherein the sensorelement is a film or a layer and comprises graphite powder.
 18. Thedevice of claim 17 wherein the sensor element comprises 5 to 20%graphite powder by weight; and wherein the graphite powder has anaverage particle size less than one micron.
 19. The device of claim 1wherein the change in the current passing through the sensor element iscontinuously monitored and displayed as a record of carbon monoxidelevels.
 20. The device of claim 1 further comprising a reference sensorelement.